Chrome-magnesia refractories



Patented June 10, 1952 2,599,566 CHROME-MAGNESIA REFRACTORIES Ralph Joseph Magri, J r., Lynchburg, Va., assignor to Corhart Refractories Company, Louisville, Ky., a corporation of Delaware No Drawing. Application June 27, 1951, Serial No. 233,945

16 Claims.

This invention relates to fused or heat-cast refractories and is particularly concerned with the provision of an improved refractory especially adapted for use in the manufacture of steel. This application is a continuation-in-part of my copending application, Serial No. 7,710 filed February 11, 1948, now abandoned and of my copendin application, Serial No. 83,999, filed March 28, 1949, now abandoned.

The use of tonnage oxygen in the open hearth furnace has recently been shown to markedly speed up steel production. Such practice, however, subjects the furnace refractories to considerably increased punishment, which in the case of the roof refractories, for example, may lower their life by as much as 50%. Corresponding improvement in such refractories is obviously imperative then if full economic advantage of the use of oxygen is to be realized. Moreover, since some five pounds of basic refractories alone are consumed for each ton of steel produced, it is also obvious that any such improved refractory must be made from raw materials common enough to be available in large quantities and cheap enough to enable such refractory to compete with present tonnage refractories in proportion to the savings obtained.

Patent No. 2,408,305 to Field describes a chromespinel composition which contains FeO, MgO, A1203, and CrzOa in a particular relationship and which, when melted at very high temperatures and then solidified in a mold, yields a very dense crystalline refractory with notable resistance to erosion by ferruginous slags in comparison to the more usual porous sintered refractories. Such chrome-spinel composition contains an appreciable amount of FeO, a feature of importance since it allows the use of a considerable percentage of commercial chrome ore in the raw batch. Unfortunately this dense refractory is less resistant to heat shock or spallin than the mor usual porous burnt refractories, which property restricts its possible places of application and in particular prevents its use as a roof refractory in an open hearth furnace where, as pointed out above, improvement is especially desirable to permit higher operating temperatures.

I have now discovered that the resistance to heat shock or spalling of such high-iron chromespinel refractory can be markedly improved, without any adverse effect on its slag resistance, by increasing its magnesia content so that, when the melted batch is solidified, a substantial amount of periclase crystallizes as a separate 2 I phase. In fact, not only must th magnesia content of the resulting refractory be suflicient so that the periclase phase is present in significant amount, but the ratio of the mols of R0 oxides to the mols of R203 oxides must be materially above unity in order to impart a satisfactory ,degree of spalling resistance to such refractory. Such ratio, I have found, should be at least 2.2 and is advantageously at least 3.5.

That my new refractories should possess such improved spalling resistance is quite surprising and unexpected since periclase itself has a higher coefiicient of expansion than the chrome-spinel composition described in the Field patent. The

coeiiicient of expansion of my refractories is not high relative to that of other refractories, however, and in some instances is even less than the expansion coefl'icients of various commercial burnt chrome and magnesite refractories.

The tensile strength of the present refractories is higher than that of commercial burnt chrome and magnesite refractories so that greater strains can be developed therein before failure. On the other hand the thermalconductivity of my refractories is also higher so that the temperature gradients obtained within such refractories when they are subjected to alternate heating and cooling are lessened. Apparently these properties are more significant with respect to spalling resistance than the dense structure of these refractories. In any event, in actual comparativ heatshock tests with panels of refractory bricks heated from 800 C. to 1400 C. at the rate of 200 C. per hour and then alternately cooled with a water spray and similarly reheated at 20-minute intervals, I have found my improved refractory compositions superior in spalling resistance to commercial burnt magnesite and chrome refractories.

As will be appreciated, relatively high temperatures are required to fuse the present compositions so that electric melting must be resorted to. The raw materials can be melted in a conventional electric furnace, the shell of which is lined with material of a composition similar to that of the refractory to be produced. The molten material can then be cast into molds. The resulting refractory pieces, following initial solidification, should be annealed either from their own heat of crystallization with the aid of appropriate insulation or with the addition of external heat in a lehr.

In the resulting fused refractory, as can be determined by analysis, the FeO is partitioned between the periclase phase, in which it is present in solid solution, and the spinel phase, in which it is present, of course, as an R spinel-forming oxide. Apparently what happens is that, as the spinel phase is formed upon cooling and solidification of the molten mass, the MgO forms spinels with the R203 oxides in preference to the FeO so that, in, effect,- theFeO is impart-displaced into th periclase phase formed by the remaining- MgO.

By reason of its dense structure, a heat-cast re-- fractory is afforded an initial advantage with respect to slag resistance over a burnt or sintered refractory, the porosity of which naturally permits a more rapid attack by slag.

The proportions of the several oxides formingthe basis of the present refractory can never-- theless be varied only within limits in accordance with the nature of the slag with which the refractory is to come in contact. While, for example, the resistance of my novel refractory to attack by basic: slag improves. with an increase in MgO -Iand; hence, the periclase phase) its: resistance to acidslagattack tends to correspondingly "decrease. Conversely, too' low an MgO content decreases the ability of the refractory to withstand basic slag attack.'.' Similarly, an increase'in crzios (and, hen'cathe chrome containing spinel phase) improves the resistance of my refractory to acid slag attack but tends to lower its resistance to basic slag. Too low a Cr-202' 'contents-0n the otherhand, decreases the resistance of-the refractory to -ac'id slag attack; A .high ratio-of CrzOa to Also: also favors acid slag resistance.- Where both types :of slag are encountered as in open hearthpractice, a practical compromise is of course-necessary The proportions within-which these severaloxides can be varied are also determined by the effect ofthe variation on the spalling resistance of the-refractoryand-by other factors. The A1203 and- CrzO; c0ntent,-for example, can not-be increased to such an extent thatthe ratio of the mols-of RO-oxides to the-mols ofRzO oxides 4 and advantageously should amount to at least 90%.

Since the heat-casting process required for the manufacture of the present refractories is inherently more expensive than the conventional sintering techniques of making refractories, it is an especially important feature of my invention that the instant refractory compositions, and particularly the preferred composition described below, require only commonly available and relatively-inexpensive raw materials, namely chrome ore and commercial calcined magnesite, for their production. 'Since,however, my refractories are falls below about 2.2 and theresistance of the refractory to spalling becomes unsatisfactory. For the same reason; the content of FeO and MgO can not be so-lowered that such ratio becomes less than 2.2. Moreover, as indicated above, sufficient'MgO mustbe present to formthe periclase phase; which, as can be readilydetermined, will amount to a minimum of approximately 15 to 20%. Aswill be apparent, a periclase phase can beformed only'if an excess of RO oxideexists beyond .that required to combine with the R203 oxidestoform'RoRzosz The presence of FeO in the-periclase phase does not appear, to adversely; affect its. slag resistance.

Additionally, as theMgO content isincreased, the. temperature required to effect fusion also increases lwithlthe resultsthat volatilization of MgO becomes more andmor'e'appreciable and the production of a commercially satisfactory and usable refractory casting becomes more and more diflicult. ;Cracking.of theirefractory, blocksor castings also occurs-toan increasingly'greater extent with very high percentages of Pet); I With ,these several considerations in mind, then,v my improved .refractory I composition comprises and'analytically; contains 5 to 25% H20, 25 to 378% MgO; 5'to' 25% A1203, and 1 2 to 50% Crz'Oa; with the ratio of the mols of FeO and MgO to the mols of A1203 and CrzOa being at least 2.2. To provide adequate slag resistance, moreover, the .total'of such oxides should amount to at least 82% of the entire refractory composition completely "meltedbefore casting, only the final chemical composition of the melted material is of. concern, and any suitable raw materials including by-products or beneficiated concentrates can be utilized in proportions which will maintain the final composition of the melt within the desired ranges.

' Chrome ore is basically chromite, which is ideally FeO.Cr-2O3. As is; well known, however, in nature appreciable MgO 'replacesFeO. and appreciable A1203 replaces Cr O3 the spinel lattice with the result that commercial chrome ores, which also contain a small amount of gangue; composed usually'of magnesium silicates, cover a relatively; wide range of compositions. Those ores high in CI2O3 are most-useful forchemical and metallurgical purposes and command correspondingly higher pricesthan' the more abundant grades, with lower CrzOacontent which are widely used for refractory purposes. Th-emore usual ranges of chrome-ore composition maybe indicated as follows 7 Per cent CrzOa u 30-50 Fe'O 12+-25 For theproductien ofsintered refractoriesi t is generally preferred to "use those chromeores higher in MgO and lower'in FeO. i have found however" that the less expensive ores, higher in FeO, are quite satisfactory for my purpose-because of the apparently harmless displacement of FeO into solid solution inthe periclasephase. It is to'be' noted, moreover, that the melting point of chromeore is high in itself and that the temperature required for melting the present compo.- sition progressively increases as the proportion of calcined magnesite to the chrome ore is increased. Since the less expensive chrome ores, of lower CI'zOs and higher FeO content, melt more easily,

'a larger addition of'magnesite can then be made,

if desired, without'increasing the melting? difficulty. 7

Since electric melting with graphite electrodes, as is the conventional practice, results in reduction of some of the F'eO and theCl'zOa as well as volatilization of some of the MgO it might be supposed thatcomposition control of theresulting refractory product would be difii'cult. I have found, however, that such losses generally balance in such a way that the percentages of Ci'2O3-and FeO in the fused product are substantially those of the batch, while there is an increase in alumina at theexpense of magnesia. It'is only necessary therefore to add an extra amount of calcined magnesite to the batch to make up for the anticipated loss, the excess required being larger as the magnesite proportion and consequently the 'melting temperature increase.

As indicated above, commercial chrome ores contain a small amount of siliceous gangue. When a chrome ore is used as a raw material in the preparation of the present refractory, then, a siliceous matrix containing any impurities is also formed upon solidification of the molten refractory composition. This siliceous matrix is largely crystalline in nature, even in relatively rapidly cooled castings. In the absence of appreciable calcium oxide, this siliceous matrix or phase has the crystallographic properties of forsterite, 2Mg0.Si02, with presumably some Fe replacing a part of the MgO.

Moderate amounts of this siliceous matrix, such as are fortunately obtained with commercial grade'chrome ores, are not harmful either to the heat-shock resistance of my refractory or to its resistance to acid slag attack. As the percentage of silica is increased, however, resistance to spalling decreases; and with larger amounts the castings crack so badly even before heating that they are unsaleable.

For this reason it is not generally feasible to use olivine, a typical analysis of which is 47%Mg0, 7% FeO, 1% Ca0, and 44% Si02, as a source of magnesia.

Moreover, as the silica content is increased, the resistance of the present refractory against acid slag decreases. From this standpoint as well as the standpoint of heat-shock resistance, therefore, the Si02 content should be not more than 8% and advantageously not more than With SiOz in the composition, the periclase phase can, of course, be formed only if there is an excess of R0 oxide beyond that required to combine with the Si02 to form 2R0.Si02 and to combine with the A1202 and the 01202 to form B03203. An excess of R0 oxide is therefore indicated when the ratio of themols of Fe0 and MgO to the mols of A1202 and Cr202 plus half the mols of Si02 exceeds unity. As previously pointed out, this ratio is at least 2.2 and advantageously at least 3.5 in the present refractory.

Commercially available calcined magnesites of good quality generally contain a small amount of CaO as an impurity. Dolomitic magnesites may also be utilized under certain conditions, but then CaO in appreciable quantities will be included in the refractory composition. Since CaO does not form a spinel, it will appear in the siliceous matrix when it is present in the raw materials. As the Ca0 increases in proportion, it progressively replaces MgO to give monticellite, dicalcium silicate, and finally tricalcium silicate. If the combining power of the Si02 is exceeded, calcium chromite is formed with the result that serious cracking of the castings occurs during their manufacture. The basic slag resistance of the present refractory is also decreased as the CaO content is increased, especially in the presence of significant quantities of Si02.

For these reasons, then, the CaO content should not be more than 10%; and advantageously no more than 3% CaO is present.

Since the effect of C210 is to displace MgO from the siliceous matrix, an excess of R0 oxide is indicated in such case when the ratio of the mols of FeO, MgO, and CaO to the mols of A1202 and Cr202 plus half the mols of Si02 (if present) exceeds unity. As already stated, this ratio is at least 2.2 and advantageously at least 3.5 in the present refractory.

Because of the use of graphite or carbon electrodes in the electric-furnace melting of heatcast refractories, it is normally expected that ferric oxide in the raw materials wilLbe reduced to ferrous oxide. I have found however that in the presence of large proportions of magnesia, relatively high concentrations of ferric oxide can be maintained in the present refractory. The moderate amounts of ferric oxide that might be obtained from the use of ferruginousbauxite or partially oxidized chrome ore or the like show no appreciable effect on the resistance to heat shock. As the R202 content is increased, however, the heat-shock resistance becomes gradually less. Moreover, if raw materials are used which permit the concentration of ferric oxide to become too high, a second spinel phase, immiscible with the chrome-containing spinel phase, results and serious cracking of the castings occurs. This additional spinel phase has the properties of magnesio-ferrite, MgOI'eM. Because of these adverse effects, the Fe202 content should. not be more than 7% and is advantageously no more than 5%.

The Fe202 initially enters the chrome-containing spinel phase as another R202 oxide and, when present, should be taken into account in determining the R0.R202 ratio. In such system therefore an excess of R0 oxide is indicated when the ratio of the mols of MgO, FeO, and CaO (if present) to the mols of A1202, Cr202, and FezOa plus half the mols of SiOz (if present) exceeds unity.

As indicated above, this ratio is at least 2.2 and is advantageously at least 3.5 in the present refractory.

The melt (casting) analyses given in Table I are illustrative of the composition of my improved refractory:

Table! Melt r00 MgO .4140, 01,01 S10, 0110 r2201 1 10.0 28.0 38.6 21.2 1.4 0.2 11 24.4 20.1 17.4 22.0 4.6 0.5 111 7. 9 2.4. 2 18.7 37. 0 2. 0 0. 2 1v 20.3 25.8 10.2 33.2 4.2 0.2 v 18.4 28.1 15.8 21.2 0.3 0.2 v1-- 10.7 20.9 12.0 28.5 5.0 0.1 v11 12.0 32.4 5.2 46.2 3.1 0.2 v1n 1s. 2 20. 7 10. 0. 21. a 2. 7 4. 2

7.7 48.0 22.8 18.2 2.4 00 XI 17. 2 32. e 0. 2 31. 0 2. 7 0. 4 2:11..-. 7. 9 53.8 10. 0 20. 0 1. 4 0. 4 x111 12.3 48.6 3.2 20.5 7.4 3.0 xiv.-. 11. 2 40.1 7. 5 20. 5 2. 4 3. 2

Each of these refractory castings comprises a chrome-containing spinel, a crystalline solid solution of ferrous oxide in magnesium oxide (periclase), and a siliceous matrix. There is only one spinel phase, which contains both FeO and MgO as R0 oxides and both Cr202 and A: (and Fe202) as R203 oxides in random distribution. Only melts IV to XV possess satisfactory resistance to spalling as well as satisfactory resistance to slag attack, however, melts I to III exhibiting poor or unsatisfactory spalling resistance. As can readily be determined (in the manner explained above), the molal ratio of R0 oxides to R202 oxides in melts IV to XV is 2.2 or greater, whereas such ratio for each of melts I to III is only 1.6, 1.9, and 2.1 respectively.

Especially good resistance to heat shock or spalling as well as especially good resistance to both acid slag and basic slag are obtained when 'my improved refractory composition comprises and analytically contains 7 to. 20% Fe0, 35 to 68% MgO, 5 to 21% A1203, and 12 to 40% CrzOa.

of FeO and MgO to the mols of A1203 and 01203 being at least 2.2.

2. The fused refractory material as claimed in claim 1, in which the total of FeO, MgO, A1203 and (H203 amounts to at least 90%.

3. A fused refractory material comprising a chrome-containing spinel, a crystalline solid solution of ferrous oxide in magnesium oxide, and a siliceous matrix, said refractory material analytically containing to 25% FeO, 25 to 78% MgO, 5 to 25% A1203, and 12 to 50% C1203, the total of such oxides amounting to at least 82%, and Si02 in an amount up to 8%, the ratio of the mols of FeO and MgO to the mols of A1203 and (M03 plus half the mols of SiO2 being at least 2.2.

4. The fused refractory material as claimed in claim 3, in which the total of FeO, MgO, A1203, and C1203 amounts to at least 90%.

5. A fused refractory material comprising a chrome-containing spinel, a crystalline solid solution of ferrous oxide in magnesium oxide, and a siliceous matrix, said refractory material analytically containing 5 to 25% FeO, 25 to 78% MgO, 5 to 25% A1203, and 12 to 50% CrzOs, the total of such oxides amounting to at least 82%, Si02 in an amount up to 8%, and OaO in an amount up to 10%, the ratio of the mols of FeO, MgO and 02.0 to the mols of A1203 and Cr203 plus half the mols of Si02 being at least 2.2.

6. The fused refractory material as claimed in claim 5, in which the total of FeO, MgO, A1203, and Cr203 amounts to at least 90%.

7. A fused refractory material comprising a chrome-containing spinel, a crystalline solid solution of ferrous oxide in magnesium oxide, and a siliceous matrix, said refractory material analytically containing 5 to 25% FeO, 25 to 78% MgO, 5 to 25% A1203, and 12 to 50% Cum, the total of such oxides amounting to at least 82%, Si02 in an amount up to 8%, CaO in an amount up to 10%, and F'e203 in an amount up to 7%, the ratio of the mols of FeO, MgO and CaO' to the mols of A1203, 01203 and Fe203 plus half the mols of Si02 being at least 2.2.

8. The fused refractory material as claimed in claim '7, in which the total of FeO, MgO, A1203, and Cr203 amounts to at least 90%.

9. A fused refractory material comprising a chrome-containing spinel and a crystalline solid solution of ferrous oxide in magnesium oxide, said refractory material analytically containing 7 to 20% FeO, 35 to 68% MgO, 5 to 21% A1203, and 12 to 40% 01203, the total of such oxides amounting to at least 90%, the ratio of the mols of FeO and MgO to the mols of A1203 and -Cr203 being at least 3.5.

10. A fused refractory material comprising a chrome-containing spinel, a crystalline solid solution of ferrous oxide in magnesium oxide, and a siliceous matrix, said refractory material analytically containing 7 to 20% FeO, 35 to 68% MgO, 5 to 21% A1203, and 12 to 40% 01203, the total of such oxides amounting to at least 90%, and Si02 in an amount up to 8%, the ratio of the mols of Fe0 and MgO to the mols of A1203 and Cr203 plus half the mols of S102 being at least 3.5.

11. A fused refractory material comprising a chrome-containing spinel, a crystalline solid solution of ferrous oxide in magnesium oxide, and a siliceous matrix, said refractory material analytically containing 7 to 20% FeO, 35 to 68% MgO, 5 to 21% A1203, and 12 to 40% C1203, the total of such oxides amounting to at least Si02 in an amount up to 8%, and CaO in an amount up to 3%, the ratio of the mols of FeO, MgO, and CaO' to the mols of A1203 and Cr203 plus half the mols of S102 being at least 3.5.

12. A fused refractory material comprising a chrome-containing spinel, a crystalline solid solution of ferrous oxide in magnesium oxide, and a siliceous matrix, said refractory material analytically containing 7 to 20% FeO, 35 to 68% MgO, 5 to 21% A1203, and 12 to 40% 0r203, the total of such oxides amounting to at least 90%, $102 in an amount up to 8%, CaO in an amount up to 3%, and Fe203 in an amount up to 5%, the ratio of the mols of FeO, MgO, and CaO to the mols of A1203, 01203 and Fe203 plus half the mols of S102 being at least 3.5.

13. A fused refractory material comprising a chrome-containing spinel and a crystalline solid solution of ferrous oxide in magnesium oxide, said refractory material analytically containing 9 to 17% FeO, 42 to 57% MgO, 5 to 15% A1203, and 15 to 30% Cr203, the total of such oxides amounting to at least 90%, the ratio of the mols of FeO and MgO to the mols of A1203 and CrzOa being at least 4.0.

14. A fused refractory material comprising a chrome-containing spinel, a crystalline solid solution of ferrous oxide in magnesium oxide, and a siliceous matrix, said refractory material analytically containing 9 to 17% FeO, 42 to 57% MgO, 5 170 15% A1203, and 15 to 30% (31203, the total of such oxides amounting to at least 90%, and S102 in an amount up to 5%, the ratio of the mols of Fe0 and MgO to the mols of A1203 and Cr203 plus half the mols of S102 being at least 4.0.

15. A fused refractory material comprising a chrome-containing spinel, a crystalline solid solution of ferrous oxide in magnesium oxide, and a siliceous matrix, said refractory material analytically containing 9 to 17% FeO, 42 to 57% MgO, 5 to 15% A1203, and 15 30% CIzOs, the total of such oxides amounting to at least 90%, Si02 in an amount up to 5%, and CaO in an amount up to 3%, the ratio of the mols of FeO, MgO, and CaO to the mols of A1203 and C1203 lus half the mols of S102 being at least 4.0.

16. A fused refractory material comprising a chrome-containing spinel, a crystalline solid solution of ferrous oxide in magnesium oxide, and a siliceous matrix, said refractory material analytically containing 9 to 17% FeO, 42 to 57% MgO, 5 to 15% A1203, and 15 to 30% @203, the total of such oxides amounting to at least 90%, S102 in an amount up to 5%, CaO in an amount up to 3%, and Fe202 in an amount up to 5%, the ratio of the mols of FeO, MgO, and CaO to the mols of A1203, (H203 and Fe203 plus half the mols of Si02 being at least 4.0.

RALPH JOSEPH MAGRI, JR.

No references cited. 

1. A FUSED REFRACTORY MATERIAL COMPRISING A CHROME-CONTAINING SPINEL AND A CRYSTALLINE SOLID SOLUTION OF FERROUS OXIDE IN MAGNESIUM OXIDE, SAID REFRACTORY MATERIAL ANALYTICALLY CONTAINING 5 TO 25% FEO, 25 TO 78% MGO, 5 TO 25% AL2O3, AND 12 TI 50% CR2O3, THE TOTAL OF SUCH OXIDES AMOUNTING TO AT LEAST 82%, THE RATIO OF THE MOLS OF FEO AND MGO TO THE MOLS OF AL2O3 AND CR2O3 BEING AT LEAST 2.2. 